1. Field of the Invention
This invention relates to curable organosiloxane compositions yielding gels suitable for use in optical filling applications. More particularly, this invention relates to organosiloxane compositions that cure to yield highly transparent gels exhibiting little timewise decline in visible light transmittance at elevated temperatures and that do not crack when subjected to thermal shocks.
2. Background Information
Organosiloxane compositions that cure by a hydrosilation reaction to produce highly transparent silicone gels are used as the optical-filling material between solar battery cells and the encasing plastic or glass and as optical-filling material between liquid-crystal display devices and the encasing plastic or glass.
At elevated temperatures, however, the silicone gels that cure by a hydrosilation reaction undergo a reduction in their visible light transmittance with the passage of time. As a result, optical components fabricated using these silicone gels exhibit a very low reliability at elevated temperatures. This problem is already known to be strongly influenced by the platinum compound added to the silicone gel compositions as catalyst, and by the ratio between the number of moles of silicon-bonded hydrogen atoms and the number of moles of alkenyl radicals that participate in the curing of these silicone gels. This knowledge has resulted in such tactics as, for example, (i) drastically reducing the concentration of platinum compound in curable organosiloxane composition used to prepare the gel to a level at which only a negligible timewise decline in visible light transmittance is exhibited by the resulting silicone gel at elevated temperatures, and (ii) the use of a large molar excess of silicon bonded hydrogen atoms with respect to alkenyl radicals in the curable composition. Unfortunately, these tactics are themselves associated with several problems. These problems include retardation of curing of the organosiloxane composition, an increase in the hardness of the resulting silicone gel with the passage of time, and foaming of the organosiloxane composition during the course of its cure.
There have also been various other proposals directed at inhibiting the high-temperature timewise decay in visible light transmittance by silicone gels cured by a hydrosilation reaction. For example, Japanese Laid Open Patent Application [Kokai or Unexamined] Number Sho 54-48720 [48,720/1979] describes an organosiloxane composition consisting of an organopolysiloxane having on average approximately 2 silicon-bonded vinyl groups in each molecule, an organopolysiloxane containing an average of at least 3 SiH in each molecule, an organopolysiloxane having in each molecule at least 1 silicon-bonded hydroxyl and at least 2 silicon-bonded vinyl groups, and a silane containing at least 1 epoxy-functional organic group and at least 1 silicon-bonded alkoxy group, and a platinum compound.
Japanese Laid Open Patent Application Number Sho 54-101884 [101,884/1979] describes an organosiloxane composition curable to a gel that consists of an alkenyl-endblocked diorganopolysiloxane (unless specified otherwise, here and below the term "endblocking" refers to both terminal positions of the molecule), an organopolysiloxane containing at least 2 SiH in each molecule, and a platinum compound.
Japanese Laid Open Patent Application Number Sho 62-39659 [39,659/1987] describes an organosiloxane composition curable to a gel. The composition contains an organopolysiloxane containing an average of from 0.1 to 2.0 silicon-bonded vinyl radicals in each molecule, an organohydrogenpolysiloxane containing an average of more than 2 SiH in each molecule, an organohydrogenpolysiloxane containing an average of less than 2 SiH in each molecule, and a platinum compound.
Japanese Laid Open Patent Application Number Sho 62-39660 [39,660/1987] teaches a composition curable to a silicone gel. The composition comprises an organopolysiloxane containing an average of from 0.1 to 2.0 silicon-bonded vinyl radicals in each molecule, a tris(dialkylhydrogensiloxy)-organosilane, and a platinum compound.
Japanese Laid Patent Application Laid Open Number Sho 62-181357 [181,357/1987] discloses a gel-forming curable organosiloxane composition containing a platinum compound, SiH-endblocked diorganopolysiloxane, vinyl-endblocked diorganopolysiloxane, and organopolysiloxane composed of the dimethylsiloxane, methylsiloxane, trimethylsiloxy, and dimethylvinylsiloxane units.
Japanese Laid Open Patent Application Number Hei 3-56565 [56,565/1991] teaches a gel-forming organosiloxane composition containing an alkenyl-functional organopolysiloxane, an organohydrogenpolysiloxane containing in each molecule on average 1 alkenyl group and 1 SiH, and a platinum compound.
Gels prepared using the organosiloxane compositions described in the preceding paragraphs still do not exhibit a satisfactory inhibition of the high-temperature timewise decline in visible light transmittance. These curable compositions also suffer from another problem. The compositions are being used in narrow gaps due to the present trend of using smaller amounts of the composition to fill the space between solar battery cells and the encasing plastic or glass and between liquid-crystal display devices and the encasing plastic or glass. However, under these conditions the silicone gels obtained from the prior art organosiloxane compositions described in the preceding paragraphs undergo cracking when subjected to thermal shock.
The present invention was achieved as a result of extensive research directed at solving the problems described in the preceding sections of this specification. In specific terms, one objective of this invention is to provide curable organosiloxane compositions that cure to yield optically transparent gels suitable for use as fillers exhibiting little timewise decline in visible light transmittance at elevated temperatures and which do not crack when exposed to thermal shock.